1. Technical Field of the Invention
This invention most generally relates to spray guns, both air atomizing and hydraulic pressure atomizing; and in particular, it relates to the shut-off/clean out needle mechanism of a spray gun and the use of a rolling wall diaphragm seal to isolate the shut-off valve needle from the needle actuator, and to quick disconnect components for servicing the front end and liquid chamber elements of the spray gun.
2. Background Art
Prior art FIG. 1 details the mechanics of a conventional, air-atomized spray gun system, through which a source of liquid to be sprayed, and a source of compressed air, are emitted from a special nozzle in converging streams so as to cause the stream of liquid to be atomized by the air flow and projected in a precise pattern; permitting the controlled spray deposition of the liquid material onto a target surface.
The spray air and fluid are directed under pressure in converging streams so as to generate the atomized spray pattern. After use, a suitable solvent is typically run through the system for cleaning, further challenging the integrity of the seals. Common practice includes frequent partial disassembly, for cleaning and closer inspection of individual components.
Conventional atomizing spray gun designs include a liquid chamber and a tip section with an axial fluid outflow orifice and shut off needle, where the needle has a tapered or conical section that contacts the back of the fluid tip to close off the flow of the liquid being sprayed. The shut off needle can also have an extended tip end clean out section of uniform diameter, where the clean out portion of the needle protrudes through the fluid tip orifice when the needle is in the closed or sealed position.
The needle travels back and forth between the fully closed position, preventing flow of the liquid being sprayed, and the retracted or open position, allowing the liquid to flow out the nozzle. A biasing force in the shut off direction, sufficient to hold the needle forward, closed and sealed at all operating pressures, is provided by a spring on the aft end of the needle shaft. The actuating force to open the orifice and start the flow of liquid is provided by a piston attached to the needle shaft. The piston is located at the aft end of an air cylinder through which the needle shaft passes. When air is applied to the cylinder, the piston compresses the spring and pulls the needle away from the nozzle to permit liquid flow.
Between the liquid chamber and the air cylinder is an interference seal through which the needle shaft slides. This seal prevents the liquid from the liquid chamber from leaking along the needle shaft into the air cylinder. The interference seal is commonly an "O" ring, quad seal or a packing bushing that is intended to be resistant to the materials being sprayed, and the solvents commonly used for clean up. This simple and economical sealing method may be satisfactory for many spraying applications, but for liquids with a high adhesion rate or liquids with a high abrasion rate, these seals are likely to fail in a relatively shorter period of time. This is due to the fact that the seal cannot adequately wipe the needle clean as it cycles, allowing some of the liquid to pass into and by the seal and thereby cause premature failure of the seal.
Referring to prior art FIG. 1, the illustrated prior art spray gun 100 includes a nozzle section consisting of a fluid tip 101 and air cap 102 secured by collar 103 to body 104. Spray liquid port 105 connects a liquid source to the liquid chamber and hence to fluid tip 101. Spray air port 106 connects through internal passageways to air cap 102. Axial metering and shut off needle 107 is operated between open and closed positions by single acting air cylinder and piston assembly 108, and shut off biasing spring 109. Needle opening port 110 connects to cylinder and piston assembly 108. Needle shaft seals 111 and 112, divided by vent 113, isolate the liquid from the front end of spray gun 100 from the air cylinder and piston assembly 108.
A close inspection of this and other prior art designs for atomizing spray guns discloses a further problem relating to how to best service the spray gun after use. The typical time required for break down, cleaning and/or maintenance, and re-assembly, is often a disincentive to good maintenance practices.
Other types of seals have been used in related devices and specialty spray guns to isolate the metering valve stopper and liquid chamber from the valve actuating mechanism. Kingsford's U.S. Pat. No. 5,695,120, is an airless spray gun for de-ionized water that provides continuous circulation of liquid from the liquid source through the gun between "on" periods of spraying. It has a valve stopper member that incorporates a diaphragm disk element for sealing the back wall of the liquid chamber from the trigger mechanism.
There are other classes of paint spraying devices. For example, airless paint sprayers utilizing direct pumping of paint with piston or diaphragm pumps are well known in the art. Such patents as Geberth's U.S. Pat. No. 4,768,932, a hydraulic paint pump; Wagner's U.S. Pat. No. 3,680,789, a spray gun with an electric vibrator and a diaphragm pump; and Juterbock's U.S. Pat. No. 5,863,184, a diaphragm pump for spraying high viscosity liquids, will provide context for some related materials.
Rolling wall diaphragms are commonly known to be used as seals in other types of piston and cylinder applications. Furon Company's WO98/02659 patent application for a double acting pneumatically driven rolling wall diaphragm pump is illustrative of this sealing technique.
In summary, there is clearly room for improvement in the art of atomizing spray guns, particularly as to the sealing of the needle shaft, and also in the provisions for disassembly and maintenance of the spray gun.